Nanoparticulate compositions, first described in U.S. Pat. No. 5,145,684 ("the '684 patent"), are particles consisting of a poorly soluble therapeutic or diagnostic agent having adsorbed onto the surface thereof a non-crosslinked surface stabilizer. The '684 patent describes the use of a variety of surface stabilizers for nanoparticulate compositions. The use of a PEG-derivatized phospholipid, PEG-derivatized cholesterol, PEG-derivatized cholesterol derivative, PEG-derivatized vitamin A, or PEG-derivatized vitamin E as a surface stabilizer for nanoparticulate compositions, or any other component of such compositions, is not described by the '684 patent.
The '684 patent describes a method of screening drugs to identify useful surface stabilizers that enable the production of a nanoparticulate composition. Not all surface stabilizers will function to produce a stable, non-agglomerated nanoparticulate composition for all drugs. Moreover, known surface stabilizers may be unable to produce a stable, non-agglomerated nanoparticulate composition for certain drugs. Thus, there is a need in the art to identify new surface stabilizers useful in making nanoparticulate compositions. Additionally, such new surface stabilizers may have superior properties over prior known surface stabilizers.
A. Lipids in Nanoparticulate Compositions
A lipid is an inclusive term for fats and fat-derived materials. It includes all substances which are (i) relatively insoluble in water but soluble in organic solvents (benzene, chloroform, acetone, ether, etc.); (ii) related either actually or potentially to fatty acid esters, fatty alcohols, sterols, waxes, etc.; and (iii) utilizable by the animal organism. Because lipids are relatively insoluble in water, but soluble in organic solvents, lipids are often referred to as "fat soluble," denoting substances extracted from animal or vegetable cells by nonpolar or "fat" solvents. Exemplary lipids include phospholipids (such as phosphatidylcholine, phosphatidylethanolamine, and cephalin), fats, fatty acids, glycerides and glycerol ethers, sphingolipids, alcohols and waxes, terpenes, steroids, and "fat soluble" vitamins A or E, which are non-cholesterol based poorly water soluble vitamins. Stedman's Medical Dictionary, 25.sup.th Edition, pp. 884 (Williams & Wilkins, Baltimore, Md., 1990); Hawley's Condensed Chemical Dictionary, 11.sup.th Edition, pp. 704 (Van Nostrand Reinhold Co., New York, 1987).
A number of U.S. patents teach the use of a charged phospholipid, such as dimyristoyl phophatidyl glycerol, as an auxiliary surface stabilizer for nanoparticulate compositions. See e.g., U.S. Pat. No. 5,834,025 for "Reduction of Intravenously Administered Nanoparticulate-Formulation-Induced Adverse Physiological Reactions"; U.S. Pat. No. 5,747,001 for "Aerosols Containing Beclomethasone Nanoparticle Dispersions"; and U.S. Pat. No. 5,718,919 for "Nanoparticles Containing the R(-)Enantiomer of Ibuprofen."
Other U.S. patents describe the use of a charged phospholipid, such as diacylphosphatidyl glycerol or dimyristoyl phosphatidyl glycerol, as a cloud point modifier for the surface stabilizer of a nanoparticulate composition to prevent particle aggregation during steam heat autoclaving. See e.g., U.S. Pat. No. 5,670,136 for "2,4,6-triiodo-5-substituted-amino-isophthalate Esters Useful as X-ray Contrast Agents for Medical Diagnostics Imaging"; U.S. Pat. No. 5,668,196 for 3-amido-triiodophenyl Esters as X-ray Contrast Agents"; U.S. Pat. No. 5,643,552 for "Nanoparticulate Diagnostic Mixed Carbonic Anhydrides as X-ray Contrast Agents for Blood Pool and Lymphatic System Imaging"; U.S. Pat. No. 5,470,583 for "Method of Preparing Nanoparticle Compositions Containing Charged Phospholipids to Reduce Aggregation"; and U.S. Pat. No. 5,336,507 for "Use of Charged Phospholipids to Reduce Nanoparticle Aggregation." None of these patents refer to the use of a PEG-derivatized phospholipid, PEG-derivatized cholesterol, PEG-derivatized cholesterol derivative, PEG-derivatized vitamin A, or PEG-derivatized vitamin E in nanoparticulate compositions, either as a surface stabilizer, cloud point modifier, or as any other constituent of a nanoparticulate composition.
B. PEG-derivatized Lipids in Pharmaceutical Compositions
Liposomes, or vesicles composed of single or multiple phospholipid bilayers, have been investigated as possible carriers for drugs. Unmodified liposomes tend to be taken up in the liver and spleen. For drugs targeted to these areas, unmodified liposomes are useful drug adjuvants. However, often the liver and spleen are not the target areas for drug delivery. This affinity for the liver and spleen limits the effectiveness of liposome-encapsulated drugs and complicates dosing. Kimelberg et al., "Properties and Biological Effects of Liposomes and Their Uses in Pharmacology and Toxicology," CRC Crit. Rev. Toxicol., 6:25-79 (1978); and Allen et al., "Stealth.RTM. Liposomes: An Improved Sustained Release System For 1-beta-D-arabinofuranosyl-cytosine," Cancer Res., 521:2431-2439 (1992). To avoid these problems, researchers have studied various ways of modifying the liposome structure to prolong circulation time. Allen, Cancer Res., 521:2431-2439 (1992).
It was discovered that one useful type of modified lipid contains polyethylene glycol (PEG). In its most common form PEG, also known as poly(ethylene oxide) (PEO), is a linear polymer terminated at each end with hydroxyl groups: EQU HO--CH.sub.2 CH.sub.2 O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 CH.sub.2 --OH
This polymer can be represented as HO-PEG-OH, where it is understood that the -PEG-symbol represents the following structural unit: EQU --CH.sub.2 CH.sub.2 O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 CH.sub.2 --
PEG is particularly useful because of its ease of preparation, relatively low cost, controllability of the molecular weight, and the ability to link to lipid by various methods. PEG is believed to act by forming a hydrophilic coat and by causing steric hindrance at the liposome surface, thus reducing liposome-serum protein interaction and liposome-RES (reticuloendothelial system) cells interaction. Yuda et al., "Prolongation of Liposome Circulation Time by Various Derivatives of Polyethyleneglycols," Biol. Pharm. Bull., 19:1347-1351, 1347-1348 (1996).
PEG-derivatized lipids are described in, for example, U.S. Pat. No. 5,672,662 ("the '662 Patent") for "Poly(Ethylene Glycol) and Related Polymers Monosubstituted with Propionic or Butanoic Acids and Functional Derivatives Thereof for Biotechnical Applications," and Yuda et al. (1996).
1. PEG-Derivatized Lipid Drug Carriers Result in Increased In Vivo Circulation Times of the Administered Drug PA1 2. PEG-Derivatized Lipid Drug Carriers Result in Decreased Toxicity of the Administered Drug PA1 3. PEG-Derivatized Lipid Drug Carriers Result in Increased Stability of the Administered Drug
PEG derivatized lipids or liposomes are referred to as "sterically stabilized" lipids or liposomes (S-lipids or S-liposomes). Allen, "Long-circulating (sterically stabilized) liposomes for targeted drug delivery," TiPS, 15:215-220 (1994). PEG attracts water to the lipid surface, thus forming a hydrophilic surface on the lipid. The hydrophilic surface inhibits opsonization of the lipid by plasma proteins, leading to increased survival times of PEG-lipid in the circulation. Opsonization refers to uptake by the cells of the mononuclear phagocyte system (MPS), located primarily in the liver and spleen. Because PEG-derivatized lipids evade the cells of the MPS, they are often called Stealth.RTM. lipid or liposomes. Lasic D., "Liposomes," Am. Scientist, 80:20-31 (1992); Papahadjopoulos et al., "Sterically Stabilized Liposomes; Pronounced Improvements in Blood Clearance, Tissue Distribution, and Therapeutic Index of Encapsulated Drugs Against Implanted Tumors," PNAS, U.S.A., 88:11460-11464 (1991). (Stealth.RTM. is a registered trade mark of Liposome Technology, Inc., Menlo Park, Calif.)
The diagram below shows a representative PEG-liposome as compared to a conventional liposome: ##STR1##
PEG-lipids are highly superior over conventional lipids as they exhibit: (1) prolonged blood residence times, (2) a decreased rate and extent of uptake into the MPS with reduced chance of adverse effects to this important host defense system, (3) dose-independent pharmakokinetics in animals and humans, and (4) the ability to cross in vivo biological barriers. Allen at 216; Yuda et al. at 1349-1351; Bedu-Addo et al., "Interaction of PEG-phospholipid Conjugates with Phospholipid Implications in Liposomal Drug Delivery," Advanced Drug Delivery Reviews, 16:235-247 (1995); and Lasic et al., "The `Stealth` Liposome: A Prototypical Biomaterial," Chemical Reviews, 95:2601-2628 (1995).
For example, it has been reported that PEG-derivatized lipids can result in a great increase in the blood circulation lifetime of the particles. Studies of doxorubicin and epirubicin encapsulated in PEG-phospholipids for decreasing tumor size and growth showed that the encapsulated drugs had a much longer half-life than free drug and are cleared much more slowly from the circulation (for PEG-phospholipid encapsulated doxorubicin, the distribution half-life was about 42 hours, in contrast to the distribution half-life of about 5 minutes for free doxorubicin). The '662 Patent; Mayhew et al., Int. J. Cancer, 51:302-309 (1992); Huang et al., Cancer Res., 526774-6781 (1992); and Gabizon et al., "A Pilot Study of Doxorubicin Encapsulated in Long-Circulating (Stealth.RTM.) Liposomes (S-Dox) In Cancer Patients," Proc. Am. Soc. Clin. Oncol. 11:124 (1992).
Similarly, Yuda et al. describe prolongation of the in vivo circulation time of PEG-derivatized lipids, such as PEG-derivatized cholesterol, PEG-derivatized succinate, PEG-derivatized phosphatides, and PEG-derivatized glycerols. The results showed that incorporation of the PEG-derivatives into liposomes appreciably increased the blood level of liposomes and correspondingly decreased the RES uptake after injection. Conventional liposomes without PEG showed low blood levels and high accumulation in the liver and spleen, suggesting that these liposomes were readily taken up by the RES. Yuda et al. at 1349.
In addition to the prolonged half-life of drugs when encapsulated in PEG-derivatized lipids, it was also determined that toxicity of the administered drug is reduced compared to that observed with administration of free drug in animals. This reduction in toxicity is likely because the PEG-liposome carrier prevents a large post-administration spike in plasma levels. Mayhew et al., Int. J. Cancer, 51:302-309 (1992).
Another way in which long-circulating PEG-lipids may enhance cytotoxic cell delivery is by protecting drugs that rapidly degrade from contact with plasma for prolonged periods. For example, in a study of mice bearing leukemia tumors, ARA-C (an unstable drug) encapsulated in PEG-derivatized phospholipids was more effective at lower doses in prolonging survival time of mice than was free ARA-C or ARA-C entrapped in conventional liposomes. Allen et al., Cancer Res., 521:2431-2439 (1992). The superiority of the PEG-derivatized phospholipid delivery system to other drug delivery systems at low doses was attributed to the greatly extended circulation time of the PEG-derivatized lipids, as well as to slow leakage rates of the drug from the carrier.
There is a need in the art for nanoparticulate compositions of poorly soluble drugs having potentially long blood pool residence times, decreased toxicity, and increased stability to increase the effectiveness of the administered drug, and for methods of making such compositions. In addition, there is a need in the art for a surface stabilizer useful in preparing nanoparticulate compositions of drugs, in which prior known surface stabilizers are ineffective. The present invention satisfies these needs.